Answer:
In-depth interview
Explanation:
Zan wants to collect considerable information about the current opinions of his ten most important customers. Zan will probably use the in-depth interview research method. With the help of in-depth interviews, Zan can know his customers in a better way. He will be having more valuable insights about his customers which he otherwise could not get with any other research method. Customers will be free to tell him anything they want to tell and mention. Customers will be talking more which will result in more valid and reliable information.
